1. Field of the Invention
The present invention relates to a moving picture image encoding device which encodes a moving picture image signal for transmission at a fixed transmission rate.
2. Description of the Related Art
Progress is being made in the development of videophone and teleconferencing with a background of the preparation of digital communications networks and the advance of digital communications technologies. One of the most important technologies of realizing the videophone and the teleconferencing is moving picture image encoding.
A moving picture image encoding device is disclosed in, for instance, U.S. Pat. No. 4,951,140. In this moving picture image encoding device, an interframe difference signal between image signals of one-frame before and after, which are input on a frame-by-frame basis, is produced and then subjected to discrete cosine transform in a DCT (discrete cosine transform) circuit. DCT coefficient data output from the DCT circuit is quantized by a quantization circuit and then coded by a variable-length coding circuit. Codes output from the variable-length coding circuit are temporarily stored in a buffer memory. The contents of the buffer memory are read at a fixed transmission rate and sent to a transmission line.
The quantization step size of the quantization circuit is controlled according to the quantity of codes stored in the buffer memory so that the buffer memory will not overflow. That is, when the buffer content of the buffer memory is not less than a preset value, the quantization step size is made large, thereby decreasing the quantity of codes generated from the variable-length coding circuit. When the buffer content is less than the preset value, the quantization step size is made small, thereby increasing the quantity of codes from the coding circuit. The occurrence of underflow of the buffer memory can be prevented by storing dummy data in the buffer memory.
In this way, the quantization step size of the quantization circuit is feedback-controlled so that the buffer content may fall below a predetermined value. The feedback control may cause an oscillatory phenomenon in which the quantization step size varies at short intervals. In case where the oscillatory phenomenon occurs, the quality of a picture reproduced at the decoding side will vary with time. That is, the picture quality deteriorates.